The biggest new feature in latest version of CruiseControl.NET are Integration Queues.
This means you can control which integrations occur concurrently. It's a very useful feature if you're integrating several branches of the same product that or lots of projects. All you have to do is to add "queue" attribute to "project" tags in your CCnet.config file and assign same queue name to projects you want to group. You can also assign different priorities to project inside the group. That's exactly what was my biggest wish for CCnet and I already implemented it at work.
I have never seen "Volunteer to Fix" feature working in CCtray until this latest version. That's also a nice feature that can minimize the panic when build fails!
So there, you have two excellent reasons to upgrade your CruiseControl.NET server!